1. Field of the Invention
The present invention relates to a power converter and an air conditioner using the same power converter, and more specifically to a power converter using a high power factor rectifier circuit and an air conditioner using the same converter.
2. Description of the Prior Art
An example of high power factor rectifier circuits is disclosed by [Thee phase high power factor converter], 1994, Lecture Papers of National Convention I.E.E. JAPAN Industry Application Society.
FIG. 1 shows this prior art high power factor rectifier circuit. In the drawing, a three-phase rectifier 9 is composed of six bridge-connected diodes D.sub.1 to D.sub.6. A three phase AC power source PS is connected to three AC input terminals of this three-phase rectifier 9 via three series-connected reactors L.sub.1, L.sub.2, and L.sub.3, respectively. To each output line of the power source PS, one end of a star connection capacitors C.sub.1, C.sub.2, and C.sub.3 is connected, respectively. Further, a chopper circuit 10, a smoothing capacitor C.sub.DC and a load resistance R.sub.L are connected in parallel to each other between two output terminals of the rectifier 9. The chopper circuit 10 is composed of series-connected two insulated-gate bipolar transistors (IGBT) TX.sub.1 and TX.sub.2 each having an inverse voltage protection diode D.sub.7 or D.sub.8. The common connection point of these two transistors TX.sub.1 and TX.sub.2 is connected to a common connection point (i.e., a neutral point) of the star connection capacitors C.sub.1, C.sub.2, and C.sub.3. Further, three impedances Z.sub.1, Z.sub.2, and Z.sub.3 intervening between the power source PS and the three reactors L.sub.1, L.sub.2 and L.sub.3 are denoted for later explanation.
Since this prior art high power factor rectifier is known, the detailed description of the operation principle thereof is omitted herein. In summary, however, the object of this rectifier circuit is to allow the input line voltage (shown in FIG. 2A) on the power source side of the reactors L.sub.1, L.sub.2, and L.sub.3 and the line current (shown in FIGS. 2B to 2D) to approach a sine wave of less distortion, respectively, by applying a high frequency signal (e.g., 10 kHz which is much higher than a frequency (e.g., 50 Hz) of the power source PS) to gates g.sub.1 and g.sub.2 of the transistors TX.sub.1 and TX.sub.2 of the chopper circuit 10, that is, by turning on and off the two transistors TX.sub.1 and TX.sub.2 alternately at the high frequency.
In this prior art high power factor rectifier, since the circuit can be formed by adding a few elements to a rectifier circuit of capacitor input type, that is, since a complicated control is not required, being different from the case of current detections or of a PWM (pulse width modulation) control, there exists a feature that it is possible to improve the waveform of each line current and the power factor, in spite of a simple circuit construction.
On the other hand, FIG. 3 shows a prior art power converter for supplying AC power to a load by using a high power factor rectifier circuit shown in FIG. 1. In this power converter, an inverter 11 composed of bipolar transistors and an AC motor 12 (as a load) are connected, instead of the load resistance R.sub.L of the rectifier circuit shown in FIG. 1. In this power converter, a DC current flowing between the rectifier 9 and the chopper circuit 10 is detected by a current detector HCT using a Hall element, and a protective circuit 21 applies a current limiting signal to a chopper control circuit 22 so that the DC detected current will not exceed a predetermined value. In addition, a DC current flowing between the chopper circuit 10 and the inverter 11 is detected by a low resistance R.sub.s, and another protective circuit 23 applies another signal to an inverter control circuit 24 so that the DC current value can be limited below a predetermined value on the basis of the detected value.
When the power converter as shown in FIG. 3 is used, it is possible to control the motor 12 at variable speed while maintaining the waveform of the input current and the power factor under excellent conditions under protection of the power converter.
In the above-mentioned prior art high power factor rectifier circuit, as far as the input current lies in such a relatively small current range as 1 to 2 [A], the input current waveform and the power factor can be both maintained under excellent conditions. However, the voltage waveform is distorted, as the input current increases.
In this connection, in the rectifier circuit shown in FIG. 1, when the input current exceeds 10 [A] on condition that the power source voltage is 100 [V]; the frequency is 50 [Hz]; the switching frequency of the chopper circuit 10 is 10 [kHz]; the capacitance of each of the star connection capacitors C.sub.1, C.sub.2, and C.sub.3 is 3300 [.mu.F]; the load resistance R.sub.L is 187.5 [.OMEGA.]; and each inductance of the reactors L.sub.1, L.sub.2, and L.sub.3 is 1.5 [mH], the input current and the phase voltage change as shown in FIG. 4. This may be due to the fact that a sort of oscillation occurs in such a way that current flows from the power source line to the load through the star connection capacitors C.sub.1, C.sub.2, and C.sub.3, so that the voltage waveform is distorted and thereby the current is also distorted by the distorted voltage. In this case, the oscillation is generated by the presence of the line impedances Z.sub.1, Z.sub.2, and Z.sub.3.
On the other hand, in the power converter shown in FIG. 3, since various circuits such as the two protective circuits; that is, since the protective circuit 21 for protecting the transistors TX.sub.1 and TX.sub.2 of the chopper circuit 10 and the protective circuit 23 for protecting the switching elements for constituting the inverter circuit 11, there exists a drawback that the circuit construction is complicated to that extent.
Further, in the prior art power converter, there exists another problem in that when the load is light, the DC voltage increases, so that the parts or elements may be broken due to over-voltage, thus an improvement being so far needed.